Distributed pressure sensors are useful to determine strength or pressure upon soft objects, e.g., to measure the interface pressures of a person sitting on a chair. For this application it is necessary for the sensor to be flexible in order to adjust to the shape of the chair curvature and to adequately measure the forces exerted. Moreover, the sensor must be thin enough as to not introduce reading errors. This type of sensor usually has a thickness comprised between 0.1 and a few millimeters. In order to measure pressure at different points on a surface, it is necessary that the sensor area of each element in the distributed sensor is as small as possible. In general, according to the number of sensor elements used, these are classified into: single sensors and sensor arrays of n×n elements. These can in turn be classified according to output signals into two (on-off) or more output sensors (analogue or digital sensors).
The performance required from flexible pressure sensors is usually less than that required from conventional rigid sensors, with measurement inaccuracies of between 5 and 10% being accepted. Flexible pressure sensors are usually made up of a series of rows and columns in matrix-type arrangements. Flexible pressure sensors of n×n sensor elements provide data on pressure distribution on n2 areas of the sensor. This data is collected in the form of an electronic signal by converting the measurement of the change in resistance provided by the sensor element into voltage or intensity. The data thus obtained is linearized in order to optimize its resolution and simplify its interpretation. In order to increase measurement precision the different sensor elements are calibrated by adjusting the corresponding gains and offsets or by establishing calibration curves. Data treated in this way allows generating two- and three-dimensional pressure maps in real time.
Amongst the different technologies that exist for developing distributed pressure sensors we can mention: the technology using piezoelectric elements and pneumatic, hydraulic, resistive and capacitive technologies. Piezoelectric technology cannot be used for static measurements due to current loss in these sensors, which makes the response signal tend towards zero with time. Sensors based on pneumatic and hydraulic technologies require very complicated assemblies and large thicknesses, which limits their application in flexible sensors. Nowadays, resistive and capacitive technologies are the most used in flexible pressure sensors.
The operating principle of resistive sensors is based on the change in electric resistance that takes place in piezoresistive materials when a force or pressure is applied upon them. In the case of capacitive sensors, these are based on the change in capacitance that occurs between two parallel plates between which there is a nonconductive elastomeric material, when a force or pressure is applied upon them. This last type of sensors has the drawback of requiring very precise and highly sensitive and stable electronics, since the changes in capacitance measured are usually less than pico faradays. In contrast, resistive-type flexible pressure sensors use very simple electronics, since changes in resistance are of several orders of magnitude and fast, which is important for arrays of many sensor elements, and hardly sensitive to electromagnetic fields (another drawback of capacitive sensors). Amongst the disadvantages of these sensors we can highlight their non-linearity and the dependence of their response to the number of cycles and the history of the sensor. Moreover, the response of these sensors usually depends on temperature and the degree of relative humidity, and they can thus show low signal stability and a lifetime that is not sufficiently long.
In general, flexible pressure sensors existing on the market have a three-layer configuration with the outer layers made in a flexible material (fabric or polymer, patents U.S. Pat. Nos. 6,155,120 and 6,501,465) that is covered with conductive lines, usually metal wires (patent applications US 2003/0173195 and WO 99/39168) or conductive paste charged with metal particles (patents U.S. Pat. Nos. 6,646,540 and 6,291,568) or carbon black (patent U.S. Pat. No. 6,597,276 and patent application WO 00/25325), and the intermediate layer is made in a pressure-sensitive material, of the conductive ink type (patents U.S. Pat. Nos. 5,652,395 and 5,838,244) or a nonconductive dielectric elastomer (patent U.S. Pat. No. 5,010,774 and patent application WO 2004/061401).
The use of polythiophenes, a family of highly stable intrinsic conductive polymers that can be processed from aqueous dispersions, as active materials for the manufacture of distributed pressure sensors has not been described in the state of the art. Patents U.S. Pat. Nos. 4,959,430 and 4,987,042 describe different procedures for preparing dispersions based on poly(ethylene-dioxy-thiophene) and patents U.S. Pat. Nos. 5,766,515 and 5,370,981 their use as a transparent electrode in electroluminescent devices and to prepare anti-static plastics, respectively.
There is still a need therefore in the state of the art for alternative large surface distributed pressure sensors that overcome the drawbacks of the state of the art.